Drug abuse in different populations varies substantially and the costs to society is immense. Correct diagnosis of abuse is a key first step in dealing with drug users' problems. Traditionally, the most objective criteria for identifying drug use is urine drug testing. In contrast to urine testing which detects drugs only for several days to a week, hair testing offers the possibility of detecting drug use that could have occurred within a period of several months. A wide variety of drugs have been detected in hair including opiates, cocaine, amphetamines, cannabinoids, phencyclidine, benzodiazepines, and barbiturates. Since hair grows at an average rate of 1.0-1.5 centimeters per month, analysis of segments of hair for drug content can reveal historical drug use dating back months to years. Some caution is necessary in interpretation of positive hair test results, however, since environmental contamination of hair can occur. A person who is drug-free, but in close vicinity to a cocaine "crack" smoker could be contaminated with cocaine condensed from the atmosphere. Differentiation of environmental contamination from actual drug-use can be problematical. In addition, there may be ethnic bias in hair testing. Darker colored hair appears to retain greater concentrations of drug than lighter colored hair. Consequently, races with predominantly black hair may exhibit a higher rate of positivity than those with lighter hair. There are many potential applications of hair testing in forensic toxicology and researchers have just begun to understand its strengths and weaknesses. Hair testing is currently under study in an attempt to resolve important pharmacologic questions regarding its validity to detect active drug use. In vivo and in vitro studies are being conducted to determine if a dose-concentration relationship exists between drug administration and concentration in hair. In addition, the time course of appearance and disappearance of drug in hair is being studied. In vitro binding studies are being conducted to determine the mechanism(s) of drug deposition. Currently, it appears that melanin, the color pigment in hair, has a major influence on the amount of drug deposition found in hair. This finding suggests that a strong color bias may exist when hair is used as a specimen for drug testing.